According to the condition, the circle passes through the points (0,0) and (2,4).
Therefore, its center lies on the perpendicular bisector of the segment connecting these points.


     Now, the idea on how to solve the problem is to find the point on the perpendicular bisector EQUIDISTANT from the given points 
          AND from the given straight line.


The midpoint between (0,0) and (2,4) is the point (1,2).

The slope of the segment connecting (0,0) and (2,4) is  = 2.
Hence, the slope of the perpendicular bisector is .

Thus the equation of the perpendicular bisector passing through (1,2) is

y - 2 = 


From the other side, the distance from the point (x,y) to the given straight line 3x - 2y = 12 is equal to

d =  =  = ,

according to the lesson The distance from a point to a straight line in a coordinate plane in this site.


Therefore, the center of the circle lies on the straight line

y - 2 = 

and is equidistant from the point (0,0) and the straight line 3x-2y - 12 = 0, which means

 = .


Thus the problem is reduced to solving this system of equations

y - 2 =         (1)     and

 = .     (2)


The setup is done. The rest is just technique.